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Feb 23, 2014, 10:34 PM
3DHOG
jcdfrd's Avatar
Quote:
Originally Posted by Knife Liddle
How can you possibly think that a fan producing 7.5lbs of thrust is making the same efflux speed as one producing 4.5lbs of thrust?

If I had a 2 ft long thrust tube that ended at 69mm, then maybe the air would slow down some by then, but that`s not what`s going on here. You need to figure the exhaust area deducting for the motor area, not just take the area of a 69mm dia circle.
ok man sorry
you dont have to repeat yourself I get it
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Feb 23, 2014, 10:36 PM
3DHOG
jcdfrd's Avatar
did you mis the part about me using Stumax's efflux calculator???????

obviously it takes into account for motor area.......I reduced it further to compensate for your larger motor
Feb 23, 2014, 10:54 PM
Lee Liddle
Knife Liddle's Avatar
It doesn`t take into account a motor that extends past the exit opening
Feb 23, 2014, 11:17 PM
Lee Liddle
Knife Liddle's Avatar
Data Correction.......

I removed the extra crutch that was in front of my fan because I thought that it might be hurting thrust a little. Well, it was. I retested with a fresh charge on the batts. The amps went up to 127a and the thrust went up to 8.5lbs.

That`s more like it.

John, I don`t have access to Stu`s chart because I don`t have what ever software that it requires.

If you don`t mind, do me a favor and plug in 8.5lbs and an exit of 60mm dia. Let`s see what that numbers is.
Feb 23, 2014, 11:34 PM
3DHOG
jcdfrd's Avatar
sorry I was not challenging anything you have said or done I was just wondering why my numbers didnt match yours
Feb 23, 2014, 11:38 PM
3DHOG
jcdfrd's Avatar
Quote:
If you don`t mind, do me a favor and plug in 8.5lbs and an exit of 60mm dia. Let`s see what that numbers is.
258.9mph
Feb 23, 2014, 11:43 PM
Lee Liddle
Knife Liddle's Avatar
Thank you
Feb 23, 2014, 11:50 PM
3DHOG
jcdfrd's Avatar
your welcome
Feb 23, 2014, 11:57 PM
Lee Liddle
Knife Liddle's Avatar
Well, it doesn`t match up with the ET data that I have, but that`s all I know, except that my Minifan at 2kw produced 225mph 7 years ago. It`s hard for me to accept that 250% of that power and almost 200% of that thrust only increases the efflux speed by 33mph.

The jet that the Minifan was on did an honest 150mph, so I think that the 225 measurement was fairly accurate.

Who knows?
Feb 24, 2014, 01:24 AM
The best in EDF since 2005
Extreme_RC's Avatar
Afaik the calculator is worked out on the exit diameter being exactly that, exit diameter without anything else but a round tube. Lee is saying his motor cone extends past the end of the outlet so if his outlet is 60mm diameter its not going to be 60mm of area.

For example 69mm outlet with a 36mm motor cone sticking out through it means your actual area is 58.86mm diameter. Plug 58.9mm and 3.41kg (7.5lb) into the SM calculator you get 248mph or 398kmh.
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Feb 24, 2014, 04:25 AM
Life begins at transition
Stu's program works on disc actuator theory, so it doesn't model anything as a tube, it's actually an infinitely thin disc. It's a first order estimate, but tends to be very accurate.

If you want to model a tube, you need to find out the boundary layer momentum thickness at the wall and the region of 'dead air' behind the motor. Then you'll have the effective exhaust area.

Clearly beyond what most people do, but it may go some way as to explaining the difference between the 'math' and 'measurements'. It's not the math is wrong - it's just not necessarily the appropriate tool for the specific situation.
Feb 24, 2014, 12:37 PM
3DHOG
jcdfrd's Avatar
the area entering the fan is the same area leaving the fan roughly 2590 or the same as 57.4mm exit of a thrust tube at 100%fsa

so plug in the numbers and I get
57.4 thrust tube exit area
7.5lbs thrust
= 254.3mph efflux

so what you guys are saying is if I whack off the thrust tube and remove it all together my efflux will go up 35-40%
254 + 40% = 350mph efflux
that is what your saying right Lee
and all you guys agree right?
ok I must be one dumb ass lol!!!
ok dont answer that haha

well I have serious doubts your efflux is that high but who the heck am I?........nobody!... so it doesnt matter. dont get mad cause I question data just let the proof be in the pudding
so when you say you got 310mph efflux or 340mph what that means to me is when you fly your racers they will easily, without any effort at all go 250mph and should be much closer to 275mph right..........
I sure wish you guys could get these in the air.... cause,.. if your gonna lay down a 250mph run right out of the gate we might as well go home and have a beer.
Last edited by jcdfrd; Feb 24, 2014 at 06:19 PM.
Feb 24, 2014, 12:49 PM
3DHOG
jcdfrd's Avatar
yesterday's best run with calculated 220mph efflux
dw pass at 35 191mph
uw pass at 50 180.8mph
dw pass at 109 191.2mph
uw pass at 124 170.6mph
dw pass at 141 185.5mph
avg 186mph

Me163 speed 2 (2 min 36 sec)
Last edited by jcdfrd; Feb 25, 2014 at 12:14 AM.
Feb 24, 2014, 12:55 PM
Formerly "Heli Copter"
Jetsom's Avatar
Quote:
Originally Posted by jcdfrd
the area entering the fan is the same area leaving the fan roughly 2590 or the same as 57.4mm exit of a thrust tube at 100%fsa

so plug in the numbers and I get
57.4 thrust tube exit area
7.5lbs thrust
= 254.3mph efflux

so what you guys are saying is if I whack off the thrust tube and remove it all together my efflux will go up 35-40%
254 + 40% = 350mph efflux
that is what your saying right Lee
and all you guys agree right?
ok I must be one dumb ass mf'er lol!!!
ok dont answer that haha

well I have serious doubts your efflux is that high but who the heck am I?........nobody!... so it doesnt matter. dont get mad cause I question data just let the proof be in the pudding
so when you say you got 310mph efflux or 340mph what that means to me is when you fly your racers they will easily, without any effort at all go 250mph and should be much closer to 275mph right..........
I sure wish you guys could get these in the air.... cause,.. if your gonna lay down a 250mph run right out of the gate we might as well go home and have a beer.
In that setup up the air may be exiting the shroud at 340mph, but without a thrust tube, the direction of force would be all over the place. I don't know what the vector of force perpendicular to the rotor would be, and that's the force that contributes to the jet's speed. IMHO
Feb 24, 2014, 01:15 PM
Lee Liddle
Knife Liddle's Avatar
Quote:
Originally Posted by Heli Copter
In that setup up the air may be exiting the shroud at 340mph, but without a thrust tube, the direction of force would be all over the place. I don't know what the vector of force perpendicular to the rotor would be, and that's the force that contributes to the jet's speed. IMHO
Well, when I run it on the bench 8.5 lbs of the force is perpendicular to the rotor or it wouldn't show 8.5lbs on the scale would it. So that much high-speed air is moving directly away from the fan exit.

John have you guys measured your thrust? It's probably less than 5 lbs and you have 220 efflux right? So why do you doubt that 8.5lbs will produce over 300 through the same size fan. It's simple physics.


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